Pin the group's final mesh-sdk 206f794e with its captured conformance files, and read the engine's superseded word on a secret move (issue 774)

The captured SDK (testdata/beside, conformance/events) is taken again at the pinned commit, as CAPTURED
says. The controller's mirror of MoveSecretAnswer gains `superseded` (mesh-host b9c24a7): the vault holds
another value, the live one, and the machine's older value is not moved and not asked again. The field
test names it. A superseded row is still counted with the moved ones: counting it apart needs a field and
its words in nodeMove, more than the one line asked.

internal/link/movesecret.go

@@ -52,6 +52,9 @@ type MoveSecretAnswer struct {
 	Secret     string `json:"secret,omitempty"`
 	Generation int    `json:"generation,omitempty"`
 	Unchanged  bool   `json:"unchanged,omitempty"`
+	// Superseded says the vault holds another value of the secret, which is the live one, and this machine's older value
+	// was not moved and is not asked again (mesh-host b9c24a7, sweep 2026-10-30).
+	Superseded bool `json:"superseded,omitempty"`
 }
 
 // MoveSecretWithin is how long the controller waits for one move: the engine asks the vault twice.